The invention relates to conveyor belt apparatus, and in particular the invention is concerned with an improvement in side plates of modular plastic conveyor belts as used in a spiral conveyer belt system.
U.S. Pat. No. 4,901,844, commonly owned with the present invention, discloses a spiral, low tension conveyor belt apparatus and system formed of plastic belt modules and having improvements relating to the driving of the belt by a driving cage or driving tower of the system. In that patent, the side plates had recesses or countersink bores for the rod heads of the connecting rods, leaving an essentially smooth surface on the outside of the side plate, for engagement with the driving cage bars. Wear of the rod heads against the cage bars was prevented. The material of the side plate was preferably of a higher friction coefficient than the plastic material forming the remaining portions of the modules. Thus, good frictional engagement was made between the belt and the driving cage, with the side plate surfaces engaging essentially flatly against the cage bars for nearly uniform and smooth driving engagement of the side plates by the bars.
The described system has proven efficient and smooth in function. However, other approaches have been suggested for providing more positive gripping engagement or "position drive" between a driving cage and plastic side plates in a plastic spiral conveyor belt system, particularly for high speed spiral conveyors. See Irwin U.S. Pat. No. 4,941,566, Roinestad U.S. Pat. No. 4,741,430 and Roinestad U.S. Pat. No. 4,852,720. The Irwin patent describes jackets or caps for the cage bars of a driving cage, which are rectangular in cross section. These caps provide grooves at the outer side of each driving cage bar. The grooves cooperate with steel connecting rod heads of a metal spiral conveyor belt. Although not every rod head becomes engaged in a cage bar groove, due to phase shifting in the spacings involved, some of the rod heads do become engaged. The rod head and groove arrangement is supposed to provide some driving assistance and establish less slippage of the spiral conveyor belt against the driving cage, which moves circumferentially faster than the belt in "overdriving" relationship.
The Roinestad patents disclose another type of "positive drive" for a spiral conveyor system. In the Roinestad patents, cage bar caps include linear vertical protrusions positioned to engage against protruding rod heads in a metal spiral conveyor belt. The vertical driving protrusions of the cage bar caps are intended to grip against the protruding belt rod heads and thus drive the belt, or a portion of the belt, at the same speed as the driving tower for a certain period or arc of movement. Since a spiral conveyor belt rises as it progresses, the rod heads in the Roinestad arrangement were to ride up on the cage bar protrusions until they were released at a vertical gap or interruption in the cage bar protrusion. This would release the rod heads and allow the rod and belt edges to spring back, then subsequently engage a second, different cage bar protrusion farther back than the first. In this way, the Roinestad "positive drive" arrangement was intended to intermittently drive groups of protruding rod heads at the same speed as the driving tower.
It is a principal purpose of the present invention to provide improved driving engagement between a spiral conveyor driving cage and a plastic conveyor belt, with structure which is relatively simple and advantageous over prior structures aimed toward the same purpose.